Disposal of nonhazardous, municipal solid waste (MSW) is becoming a substantial problem across this country as well as the world, since landfill space is becoming more limited and regulations are forcing many landfill sites to close. At the same time costs for disposing of municipal trash are increasing. Incineration of municipal trash is a method which many areas and municipalities employ for dealing with this problem, at least partially, since incineration reduces the volume of the MSW by about 90% and the weight by about 70% , while at the same time eliminating any biologically active materials. Additionally, the energy produced in the incineration can be utilized for generating steam and electricity. This is a great savings in the volume of material to be placed in a landfill and in the weight of material to be transported to a landfill. However, the incineration of MSW tends to concentrate its metals content, particularly heavy metals such as lead and cadmium, in the residue or ash from the combustion. Thus, while incineration reduces the volume of material to be landfilled, at times it tends to produce a residue or ash which contains concentrations of heavy metals in a form, usually a halide, that can leach upon contact with groundwater. When this occurs, such materials become unsuitable for traditional municipal landfill sites and special landfills and/or secondary ash treatment procedures are required.
The ash from the incineration of MSW or the combustion of other carbon-, heavy metal-, and halogen-containing materials which remains behind in the combustion zone and usually falls to the bottom of the combustion zone (Bottom Ash) makes up over 85% of the residual solids generated by incineration or combustion. Generally, Bottom Ash tends to have lead and cadmium concentrations of less than about 2500 ppm and 15 ppm by weight, respectively. Usually, the metals in Bottom Ash are not in a soluble form, such as a chloride, and, therefore, Bottom Ash is typically innocuous and poses no environmental burden. In fact Bottom Ash can even be used beneficially, such as an aggregate.
On the other hand, however, the very small, solid, particulate matter which is usually carried out of the combustion zone by exiting gas, such as flue gas, makes up the remaining residual solids generated in MSW incinerators or other combustion zones. This entrained solid particulate matter is generally termed Fly Ash. It is enriched in lead and cadmium halides, particularly chlorides, and upon separation from the gas in which it is entrained can represent a disposal problem since, upon exposure to ground water (such as in a landfill) can leach substantial quantities of lead and cadmium. Typical Fly Ash from an MSW incinerator can have lead and cadmium concentrations of greater than about 3500 ppm and 200 ppm by weight, respectively. Thus, while incineration reduces the total volume and weight of material for disposal, it produces a material which can present a disposal problem.
Several methods have been suggested to stabilize these residual solids to prevent the leaching of heavy metals, such as lead and cadmium, into groundwater. U.S. Pat. No. 4,629,509 teaches the addition of calcium sulfide to the fly ash produced from incineration of MSW in order to form highly insoluble cadmium and lead sulfides, thereby immobilizing the lead and cadmium and preventing their leaching. This patent also suggests effecting heavy metal stabilization through addition of a mixture of lime and an aqueous solution of a soluble sulfide such as sodium sulfide.
It has also been suggested, in U.S. Pat. No. 4,737,356, that the addition of a water soluble phosphate to ash containing free lime immobilizes the lead to leaching in a pH range of from approximately 5 to 12. Soluble phosphate addition in the form of phosphoric acid in the proportion of from 1 to 8% by weight of the ash is taught to reduce the leachable lead to below the EPA regulatory limits over a broader pH range than without this treatment.
Another method suggested for stabilizing waste materials, ash and related residues is through addition of soluble silicates and silicating setting agents to the waste material in order to produce insoluble metal silicates. This technique is described, for example, in U.S. Pat. No. 3,837,872.
It has further been suggested (U.S. Pat. No. 4,299,611) that ash may be vitrified in a glass furnace at a temperature in the range of about 2500.degree. F. The resulting glass material, which has a significantly reduced surface area, is said to resist extraction of the heavy metals when exposed to groundwater or to EPA tests designed to simulate groundwater extraction conditions.